1. Field of the Invention
This invention relates to a method for fabricating a field emission type emitter suitable for use in, for example, fabrication of a flat display using a field emission type emitter.
2. Description of the Related Art
Conventional flat displays using a Spindt type field emission type emitter are constructed, for example, as shown in FIGS. 1 and 2. That is, a plurality of parallel strip-shaped cathode electrodes 102 and a plurality of parallel strip-shaped gate electrodes 103 are provided on a rear glass panel 101 in a crossing relation via an insulating film 104. The gate electrodes 103 have formed tens to thousands holes 103a at each location overlapping the cathode electrodes 102. The insulating film 104 has cavities 104a at portions underlying the holes 103a to expose the cathode electrodes 102, and the cathode electrodes 102 have formed conical cathodes 105 for emitting electrons in locations exposed into the cavities 104a of the insulating film 104. On the other hand, a front glass panel 106 is provided in confrontation with the rear glass panel 101. the front glass panel 106 has formed, on its surface facing to the rear glass panel 101, fluorescent panels 107 which are sized and shaped identically to the electron lead-out electrodes, namely, the gate electrodes 103.
The principle of operations of the flat display is explained below. An electric field intensive enough to emit a tunneling current is applied between the gate electrodes 103 and the cathode electrodes 105 so that electrons be emitted from the cathode 105 and hit the fluorescent panels 107 on the front glass panel 106 to produce cathode luminescence.
The field emission type emitter having a great role in a flat display is manufactured in a process explained below.
First, as shown in FIG. 3, a chrome (Cr) film 108 of a thickness from one hundred to hundreds of nm is formed on the rear glass panel 101. Next, as shown in FIG. 4, the Cr film 108 is patterned into strips by photolithography and etching to form cathode electrodes 102. Next, as shown in FIG. 5, an insulating film 104 made of SiO.sub.2 to a thickness of hundreds of nm to some .mu.m is formed to cover the cathode electrodes 102 and to smooth the surface. After that, molybdenum (Mo), tungsten (W), niobium (Nb), or the like, is stacked to a thickness of one hundred to hundreds of nm on the insulating film 104 to form a metal film, and the metal film is patterned by photolithography and etching into the form of strips to form the gate electrodes 103 crossing the cathode electrodes 102 as shown in FIG. 6. Next, as shown in FIG. 7, photolithography is done to form on the gate electrodes 103 and the insulating film 104 a resist pattern 109 having apertures 109a at crossing points in intervals of tens of .mu.m to some .mu.m. FIG. 7 is an enlarged cross-sectional view of the part for the cathode, and so are also FIGS. 8 and 12. Next, as shown in FIG. 8, using a resist pattern 109 as a mask, the gate electrodes 103 are etched to make holes 103a.
Next, as shown in FIG. 9, using the resist pattern 109 and the gate electrode 103 as a mask, the insulating film 104 is etched by wet etching to make cavities 104a. After that, the resist pattern is removed.
Next, as shown in FIG. 10, aluminum (Al), or other metal, readily detached or solved in a later step, is stacked by vapor deposition from a direction aslant of the surface of the rear glass panel 101. After that, Mo or other metal is vapor-deposited from a direction normal to the rear glass panel 101. When the metal is stacked to a certain thickness, the metal film 111 stacked on the separation layer 110 extends continuous over the cavities 104a as shown in FIG. 11, and an appropriately sized conical cathode 105 is formed on each cathode electrode 102 in the cavity 104a.
After that, as shown in FIG. 12, the separation layer 110 is removed together with the overlying metal film 111 by etching to complete the intended field emission type emitter.
The above-explained conventional method for emitting a field emission type emitter has the merit of self-forming function in which the shape of the cathode 105 is determined approximately by the ratio between the diameter and the depth of the hole 103a, namely, the aspect ratio.
However, it is difficult to stack Mo or other metal with a uniform thickness in the vertical direction on the cathode electrodes 102 within the cavities 104a. Therefore, if field emission type emitters made in the above-explained process are used to make a display of a practically acceptable screen size, they inevitably invite significant deterioration of the quality of images of the display.
A solution would be the use of a film-making apparatus promising a uniform vertical thickness of Mo or other metal. However, such a film-making apparatus needs enormous investment, and nevertheless involves practically unacceptable drawbacks, such as useless consumption of most part of an expensive metal, such as Mo, because the metal once stacked on the separation layer 110 must be removed upon making the cathode 105.